Add CONTRIBUTING.md for reminders for issues

It would be good to use the [GitHub feature for contributing guidelines](https://github.com/blog/1184-contributing-guidelines) so we get a good amount of information in the initial message. 

Things like: 
* gem version (or commit) of the related projects
* ruby version
* operating system
* code for reproduction

This could be the place to point current issues with Celluloid. 
For example: 
* Broken features: like Pool scheduling
* Broken dependencies: like jruby 1.7.3
* Scary features: like over-use of `#exclusive`